The present invention relates to a voltage controlled oscillator circuit used in communication units such as television sets and radio units and to an automatic fine tuning circuit for television.
A conventional voltage control oscillation circuit for television, for example, is comprised of a coil L and a capacitor C as shown in FIG. 8, so that its oscillation frequency is determined by constants of the coil L and capacitor C. Transistors, resistors and other components that form the circuit are fabricated integrally on a printed circuit (PC) board; however, the coil L and capacitor C that determines the oscillation frequency are mounted externally to the PC board.
On the other hand, a conventional automatic fine tuning circuit for television has an amplifier 41 for amplifying a TV intermediate frequency (IF) signal; a phase frequency comparator 42 for outputting an error signal corresponding to a phase frequency difference between the TV intermediate frequency from the amplifier 41 and a reference frequency; a voltage controlled oscillator circuit 43 for generating the reference frequency; and an amplifier 44 for amplifying the error signal, as shown in FIG. 9, wherein a TV tuner oscillation circuit 45 is automatically adjusted so that the oscillation frequency is kept constant by the error signal amplified by the amplifier 44.
However, with the conventional voltage controlled oscillator circuit of FIG. 8, because it is necessary to externally mount on the PC board the coil L and capacitor C that determine the oscillation frequency, two additional connection pins are required for circuit integration, which poses an impediment to miniaturization of the PC board. In addition, unwanted emission from the IC connection pins and coil L has adverse effects on an audio signal tuner and so on, and as such, a shield and pattern design for preventing such effects is considerably complex.
With the conventional automatic fine tuning circuit for TV shown in FIG. 9, because it is necessary to externally mount coil L and capacitor C in the voltage controlled oscillator 43 that generates the reference frequency, as in FIG. 8, the coil must be exactly adjusted to keep AFT (automatic fine tuning) accuracy to below 0.2% at worst, thus requiring a burdensome adjustment process. Also because the coil L is externally mounted, unwanted emission from the coil L and IC connection pins has adverse effects on the audio signal tuner and so on, thereby resulting in beat interference on certain channels. It would also be considerably difficult to design a shield for the tuner and a pattern for the PC board to prevent such adverse effects.
As such, the inventor attempted to contemplate a means for automatically controlling the oscillation frequency of a TV tuner oscillation circuit 45 to a constant level, wherein a TV intermediate frequency signal of an amplifier 41 is directly divided by a divider into a reference frequency f.sub.ref and inputted into a phase frequency comparator 42, so that an error signal from the phase frequency comparator 42 is used to maintain constant the oscillation frequency of the TV tuner oscillation circuit 45. In this case, however, when a crystal oscillation circuit is used as a means to generate a reference frequency f.sub.ref, a value of the crystal becomes specific in order to simplify the divider, which results in a higher cost. On the other hand, a standard crystal such as 3.579454 MHz (standard for a color signal) is used, the resulting division ratio is greater, which increases the scale of the circuit. Especially, when the intermediate frequency is 58.75 MHz (NTSC in Japan), it was found that high-speed logic (such as ECL) must be used, which would result in increased circuit current.
The first purpose of the present invention is to provide a voltage controlled oscillator circuit that completely eliminates the need for any externally mounted coil and capacitor. The second purpose of the present invention is to provide an automatic fine tuning circuit for television with low cost and high accuracy by use of a voltage controlled oscillator circuit that eliminates the need for any externally mounted coil and capacitor.